docs: def _trim(self):
          """
          Removes oldest backups that exceed the limit configured in backup_count
          option.
  
          Does not write back to file system, make sure to call self._write()
          afterwards.
          """
          index = self._get_index()
          backup_limit = config().backup_count() - 1
  
          for changeset in index[backup_limit:]:
              self.delete(changeset[0], p_write=False)